In the field of wireless communications, MIMO (Multiple Input Multiple Output) circuits are increasingly used in order to increase the capacity of the transmission channels and to improve the operation of the entire system. The use of MIMO circuits generally leads to an increase in the number of antennae to be realized for the same board. On the other hand, in order to facilitate the integration of the circuits, the antennae are now printed directly onto the PCB (printed circuit board). However, by application of the laws of physics, the length of the antennae and the length of the isolation elements depend on the wavelength. Thus, when a short-circuited slot is used to reduce the mutual coupling between two PIFA type antennae, the wavelength of the slot which must be substantially equal to λg/4 where λg is the guided wavelength at the operating frequency, gives a physical length of around 23 mm for WiFi operation in the 2.4 GHz frequency band. This length is not negligible with respect to the cost and size constraints on the printed circuit boards used in mass production. However, printed circuit boards are most often formed by substrates having a multilayer structure.